Fix spec example: Plain text response with headers

[tfesenko]

We have two different representations of the Header Object is the specification:

• Plain text response with headers defines the type directly in the Header Object:

description: The number of allowed requests in the current period
type: integer

• Headers Object Example puts the type inside the schema element:

description: The number of allowed requests in the current period
schema:
  type: integer

I believe only the latter is correct as 1) The Header Object follows the structure of the Parameter Object, 2) Parameter Object has schema property which defines the type.
=> This PR updates the "Plain text response with headers" example by moving the header's type into the schema object.

[webron]

Good catch! Thanks for the fix.